It is already known that taking pills that combine estrogen and progestin — the most common type of hormone therapy — can increase breast cancer risk. But women who no longer have a uterus can take estrogen alone, which was thought to be safe and possibly even slightly beneficial in terms of cancer risk.

The new study suggests otherwise, if the pills are used for many years. It tracked the health of about 60,000 nurses and found that use of any kind of hormones for 10 years or more slightly raised the chances of developing breast cancer.

"There's a continued increase in risk with longer durations of use and there does not appear to be a plateau," said study leader Dr. Wendy Chen of Brigham and Women's Hospital in Boston.

The hormone picture has been confusing, and the absolute risk of breast cancer for any woman taking hormone pills remains small. Doctors say women should use the lowest dose needed for the shortest time possible.

"It's hard to be surprised that if you keep taking it, sooner or later it's going to raise risk," said Dr. Robert Clarke of Georgetown University's Lombardi Comprehensive Cancer Center.